Many electronic devices currently use power factor correction (PFC) circuitry in electric transmission to reduce transmission losses and improve voltage regulation at the load.
With reference to FIG. 1, a conventional electronic ballast with a PFC capability generally has a ground (GND), a load (14), a bridge rectifier (11), a PFC circuit (12) and an inverter (13).
The load (14) may be a cold cathode fluorescent lamp (CCFL) or another type of compact fluorescent lamp (CFL).
The bridge rectifier (11) is connected to an external alternating current (AC) power source (10) (i.e. a line voltage used in a house), rectifies an AC voltage to a full-wave rectified voltage and may be implemented with four diodes.
The PFC circuit (12) improves the power factor of a light fixture and is designed generally in a boost topology to output a high direct current (DC) voltage, commonly 400 volts, and comprises a filter inductor (121), a flyback diode (122), a filter capacitor (123), a switch (120) and a PFC controller (124).
The filter inductor (121) resists changes in current and has an input end and an output end. The input end of the filter inductor (121) is connected to the bridge rectifier (11).
The flyback diode (122) conducts current in only one direction and has an anode and a cathode. The anode of the flyback diode (122) is connected to the output end of the filter inductor (121).
The filter capacitor (123) has a positive terminal and a negative terminal, is mounted between the cathode of the flyback diode (122) and ground (GND) and provides a route to ground (GND) for any alternating component in the direct current. The positive terminal of the filter capacitor (123) is connected to the cathode of the flyback diode (122). The negative terminal of the filter capacitor (123) is connected to ground (GND).
The switch (120) controls loops of the PFC circuit (12) and has two ends. One end of the switch (120) is connected to the filter inductor (121) and the flyback diode (122). The other end of the switch (120) is connected to ground (GND).
The loops are controlled by the switch (120) and comprise an ON-state loop (100) and an OFF-state loop (101).
The ON-state loop (100) is formed between the filter inductor (121) and ground (GND) when the switch (120) is closed.
The OFF-state loop (101) is formed from the filter inductor (121) through the flyback diode (122) to the filter capacitor (123) when the switch (120) is open, which charges the filter capacitor (123).
The PFC controller (124) generates a control signal to control the switch (120), which changes the relative phase between voltage and current of voltage source (10).
In this example the inverter (13) is a DC to AC converter that coverts DC voltage from the PFC circuit (12) to AC voltage that drives the load (14).
With further reference to FIG. 2, a ripple voltage (20) occurs when the full-wave rectified voltage (coming from the bridge rectifier (11)) approaches zero volts. The filter capacitor (123) of the PFC circuit (12) must have enough capacitance to supply sufficient energy to the inverter (13) to drive the load (14) during these times of low input line voltage
A filter capacitor (123) used in the conventional PFC circuit has a large capacitance and is usually an electrolytic type capacitor. The electrolytic type capacitor has a benefit of a high capacitance, can withstand high voltages and is reasonably priced for many applications.
Since the electrolytic type capacitor is composed of some type of liquid, one major drawback of the electrolytic type capacitor is its limited lifetime. The liquid will dry out over time, especially in a high temperature environment (often found in lighting applications) that significantly accelerates reduction of lifetime (the life time of the electrolytic type capacitor will reduce 50% for every 10 degrees C. of temperature increase). Generally, lifetime of an electrolytic type capacitor is 2,000 to 8,000 hours, but the lifetime of a CCFL is more than 50,000 hours.
Other capacitor technologies exist, in particular a Mylar capacitor, which has a longer lifetime and good performance for lighting applications. Unfortunately Mylar capacitors are much more expensive than the electrolytic type capacitor and 20 times larger than the electrolytic type capacitor with similar capacitance.
However, people skilled in art know that not only electronic devices using PFC circuitry have a filter capacitor. Other types of devices that can be controlled by pulsed signals may also have a filter capacitor. Therefore, all manufactures and designers are eager to lower the value/size of the filter capacitor so that it may be replaced with a long life capacitor in order to reduce manufacturing costs and increase the lifetime of their products.